Overview
This lecture explains how synapses function as communication links between neurons, detailing the processes of electrical and chemical signaling, the role of neurotransmitters, and how drugs like cocaine affect synaptic transmission.
Synapse Structure and Function
- Synapses are tiny junctions between neurons where communication occurs.
- The human brain contains 100 to 1,000 trillion synapses.
- Synapses enable learning, memory, and are linked to many psychiatric disorders.
- Each synapse can adapt, strengthen, or weaken based on activity.
Types of Synapses
- Electrical synapses send fast ion currents directly via gap junctions for quick, synchronized responses (e.g., heart muscle cells).
- Chemical synapses use neurotransmitters to send messages across the synaptic cleft, allowing for more control and selectivity.
- Chemical synapses are more common than electrical synapses in the body.
Chemical Synapse Transmission Steps
- The presynaptic neuron sends signals through the presynaptic terminal filled with neurotransmitter vesicles.
- The postsynaptic neuron has receptors to receive neurotransmitters, usually on dendrites or the cell body.
- The synaptic cleft is the small gap neurotransmitters cross.
- An action potential triggers calcium channels, causing vesicles to release neurotransmitters into the cleft.
- Neurotransmitters bind to postsynaptic receptors, opening ion channels and generating excitatory or inhibitory responses.
- Net effect of all synaptic inputs determines if the postsynaptic neuron fires its own action potential.
Neurotransmitter Roles and Drug Effects
- There are over 100 types of naturally-occurring neurotransmitters, each regulating different bodily and mental functions.
- Neurotransmitters are quickly removed by reuptake, enzymatic breakdown, or diffusion after signaling.
- Drugs like cocaine block neurotransmitter reuptake (especially dopamine), causing excess stimulation, euphoria, and eventual neurotransmitter depletion.
- Prolonged drug use reduces receptors and disrupts normal synaptic function, leading to addiction and dysfunction.
Key Terms & Definitions
- Synapse — Junction where neurons communicate via electrical or chemical signals.
- Presynaptic Neuron — The neuron sending the signal.
- Postsynaptic Neuron — The neuron receiving the signal.
- Neurotransmitter — Chemical messenger that transmits signals across the synaptic cleft.
- Action Potential — Electrical impulse that travels along a neuron.
- Reuptake — Process of reabsorbing neurotransmitters back into the presynaptic neuron.
- Gap Junction — Channel allowing direct electrical communication between cells.
Action Items / Next Steps
- Review the previous episode to understand action potentials.
- Study diagrams of electrical and chemical synapses for better visualization.
- Prepare to learn about neurotransmitter functions and related disorders.